Radio frequency (RF) transmitters are used in basestations, cellular handsets, smartphones, tablets, laptops, etc. to transmit voice and/or data to RF receivers. In general, as illustrated in FIG. 1 an RF transmitter 100 is comprised of three primary components: a modulator 102, a power amplifier (PA) 104, and an antenna 106. The modulator 102 serves to modulate the voice or data to be transmitted onto an RF carrier having a frequency capable of ‘carrying’ the voice or data over the air to a remote receiver, such as a cellular base station, Wi-Fi hotspot, or Bluetooth receiver, as the case may be, and the PA 104 operates to increase the RF power of the modulated RF carrier, just prior to being radiated by the antenna 106, in order to compensate for attenuation the RF carrier experiences as it propagates to the receiver.
In the interest of maximizing spectral efficiency, i.e., the data rate per given allocation of the radio frequency RF spectrum, many modern communications systems employ what are referred to as “non-constant envelope” modulation schemes, in which the modulator 102 modulates both the amplitude and angle (phase or frequency) of the RF carrier, in order to convey information. So that the PA 104 does not clip the signal peaks of the amplitude-modulated RF carrier as it translates the RF carrier to higher power, the PA's 104's output RF power must be backed off. The amount of back off required depends on the particular modulation scheme being used or, more specifically, on the peak-to-average power ratio (PAPR) of the modulated RF carrier resulting from application of the particular modulation scheme. Unfortunately, application of many modern non-constant envelope modulation schemes produce an RF carrier with a high PAPR, even greater than 6 dB, so the need to back off the PA's output power can be significant and the efficiency of the PA and efficiency of the RF transmitter as a whole both suffer dramatically as a consequence.
One way to avoid having to back off the output power of a PA in situations where a non-constant envelope modulation scheme is being used is to employ what is known as a “polar modulator.” In a polar modulator, the information to be transmitted, is first converted from rectangular or Cartesian (in-phase (I), quadrature (Q)) coordinates to polar coordinates ρ=(I2+Q2)1/2, ϕ=tan−1(Q/I) and modulation is then performed in the polar domain, i.e., instead of using rectangular (Cartesian) coordinates. FIG. 2 is a drawing showing the salient elements of a “digitally-intensive” polar modulator 200, which due to its near all-digital construction can be fully implemented in a single integrated circuit (IC) chip. The digitally-intensive polar modulator 200 comprises a digital signal processor (DSP) 202, a phase modulator 204, an amplitude control word (ACW) generator 206, and a digital PA (DPA) 208. The DSP 202 serves to convert input rectangular-coordinate I and Q digital data into digital polar-coordinate amplitude modulation (AM) and phase modulation (PM) signals. The phase modulator 204 modulates the RF carrier in accordance with the PM signal, to produce a constant-envelope phase-modulated RF carrier, which is then applied to the RF input of the DPA 208. Meanwhile, the AM signal is directed to the ACW generator 206, which responds by producing an ACW signal. The DPA 208 comprises a plurality of switch-mode PAs, typically Class D, E, or F switch-mode PAs, that can be connected in parallel and which are activated or deactivated depending on the ACW signal. Amplitude modulation of the phase-modulated RF carrier is therefore effected by simply activating or deactivating the switch-mode PAs in the DPA 208 according to the ACW signal. This capability combined with the fact that phase-modulated RF carrier applied to the RF input port of the DPA 208 has a constant envelope avoid having to back off the output RF power in order to prevent signal peak clipping. In non-polar architectures that employ non-switch-mode PAs or so-called “linear” PAs, such as Class A, B, and AB PAs, amplitude modulation can only be performed by passing the AM through the RF input port of the PA so, unfortunately, there is no other recourse but to back off the linear PA's output power.
Other efficiency enhancing approaches, like the more recently introduced digital polar Doherty and digital outphasing PA architectures, also exploit the high efficiency capabilities of switch-mode PAs. However, because switch-mode PAs are highly nonlinear devices, in all of these approaches some form of PA linearization is required. Often, digital predistortion (DPD) is used to counter the switch-mode PA's nonlinearities. Regardless of the PA linearization approach that is used, however, the linearity of the PA and the linearity of the RF transmitter as a whole are both significantly impacted by the RF transmitter's phase modulator. As the phase modulator upconverts the information to be transmitted to RF, it produces local oscillator (LO) harmonics at multiples of the RF carrier frequency (LO frequency). The harmonics manifest as distortion in the output RF spectrum of the PA and tend to interfere with adjacent channels, making it difficult to comply with noise limitation requirements imposed by communications standards. The 3rd-order harmonic is particularly problematic since it is closest to the fundamental (desired) LO frequency and because it can intermodulate with the fundamental component when passing through the PA. The resulting third-order counter-intermodulation distortion (C-IMD3) can undesirably fall very near and sometimes even within the intended transmission band. Typically, a band-pass filter (BPF) 210 is used to filter out the harmonic distortion and the C-IMD3. However, BPFs are not always effective, constrain the frequency agility of the RF transmitter, are bulky, and include inductors, which are particularly undesirable in fully integrated implementations since they require large IC areas.
In addition to being the source of undesirable harmonic distortion, conventional phase modulators are often inaccurate, have a limited modulation bandwidth, and/or are capable of operating only over a very narrow range of frequencies, i.e., lack frequency agility themselves, regardless of whether a BPF is present.
Considering the drawbacks and limitations of conventional phase modulators, it would therefore be desirable to have a phase modulator for a polar modulator that: 1) is digitally implemented; 2) has a wide modulation bandwidth; 3) is frequency agile; and 4) is capable by itself of preventing LO harmonics from being produced at its output, particularly 3rd-order and 5th-order harmonics.